Update documentation to show how to use new GraphQLKickstartContext

[kbrooks]

Is your feature request related to a problem? Please describe.
A new GraphQLKickstartContext object was introduced in graphql-java-kickstart/graphql-java-servlet#410 but the dataloaders documentation still refers to the old object. https://www.graphql-java-kickstart.com/servlet/dataloaders/

Describe the solution you'd like
Update the documentation to refer to the new GraphQLKickstartContext object

Describe alternatives you've considered
n/a

Additional context
n/a

[federicorispo]

@kbrooks Thanks for your time. We will update the documentation adding the usage of the GraphQLKickstartContext.

I move this in the documentation repository.